Kinetics and products from reaction of Cl radicals with dioctyl sebacate (DOS) particles in O2: a model for radical-initiated oxidation of organic aerosols

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Abstract
The reaction of Cl radicals with bis (2-ethylhexyl) sebacate (also known as dioctyl sebacate, DOS) particles in the presence of O2 is studied as a model of radical-initiated oxidation of organic aerosols. The uptake coefficient as measured from the rate of loss of DOS is γDOS = 1.7 (±0.3) indicating that a radical chain is operative. It is observed that nearly all of the detected products, accounting for 86% (±12%) of the reacted DOS, remain in the particles indicating that they are not efficiently volatilized. Correspondingly, the particles do not decrease in volume even after 60% of the DOS has reacted; upon further reaction the volume does decrease by up to 20%. Additionally, the mass of a DOS film increases with reaction indicating that the density increases. The two primary products identified are the ketone (38 ± 10% yield) and alcohol (14 ± 4% yield) resulting from reactions of alkylperoxy radicals originating from DOS oxidation. The fact that the ketone/alcohol ratio is >1 implies that the Russell mechanism, the typical fate of alkylperoxy radicals in liquids whereby both a ketone and an alcohol are generated, is not the only source of ketones. In fact, the ketone yield demonstrates a Langmuir–Hinshelwood type dependence on the O2 concentration indicating that 44% (±8%) of the ketone is created from the reaction of alkoxy radicals with O2 at the surface of the particles (at 20% O2). While this is a common reaction in the gas phase, it is generally not considered to occur in organic solvents. Furthermore, the appearance of gas-phase H2O2 suggests that peroxy radicals react to form two ketones and H2O2via the Bennett and Summers mechanism. The absence of aldehyde products, both in the gas phase and in the particles, indicates that β-scission of the alkoxy radicals is not significant. The results of this study suggest that organic aerosols in the troposphere are efficiently oxidized by gas-phase radicals but that their chemical transformation does not lead to their removal through volatilization.